Circuit boards that carry electronic integrated circuits as well as discrete electronic components are well known. A circuit board substrate is prepared with predetermined conductor paths and pads for receiving the leads of electronic components such as integrated circuit chips, resistors or capacitors. During the circuit board fabrication process, solder paste bricks are placed onto the board substrate at appropriate positions. The solder paste bricks are usually applied by placing a screen onto the substrate, applying solder paste through the screen openings and removing the screen from the substrate. The circuit board electronic components are then positioned onto the substrate, preferably with a pick and place machine, with leads of the electronic components placed on the respective solder paste bricks. The circuit board is passed through an oven after all of the components are positioned on the substrate to melt the solder paste thus creating an electrical as well as mechanical connection between the components and the substrate.
The size of the solder paste bricks and the accuracy with which they must be placed on the substrate has become increasingly smaller and tighter with the increased emphasis on miniaturization in the electronics industry. Solder paste brick heights can be as small as 100 microns and the height of the solder paste brick must often be measured to within 1 percent of the designed height and size. The center-to-center spacing between solder bricks is sometimes as little as 200 microns. Too little solder paste can result in no electrical connection between the lead of an electronic component and the pad of the circuit board substrate. Too much paste can result in bridging and short-circuiting between the leads of a component.
A single circuit board can cost thousands and even tens of thousands of dollars to manufacture. Testing of a circuit board after the fabrication process is complete can detect errors in solder paste placement and component lead connection, but often the only remedy for a faulty board is rejection of the entire board. It is accordingly imperative that a circuit board be inspected during the fabrication process so that improper solder paste placement can be detected prior to the placement of the electronic components onto the substrate. Such in-process solder inspection reduces the cost of failure since expensive components have not yet been placed onto the circuit board.
Current solder paste inspection systems that employ phased profilometry have some limitations. The use of white light phased profilometry is a well known for optically acquiring topological surface height data. Typical phase profilometers used to acquire height topologies of test surfaces generally use triangulation principles combined with structured light to determine the height of the surface at every point defined by the sensor's imager. One limitation of using triangulation sensing to produce a height image of a test surface is that the source and receive optics use separate optical axes. If the test surface has height features that have an edge slope large enough that they occlude either the source or receive optical path relative to some area on the surface, the sensor will not be able to measure those areas of the surface.
One approach to mitigate the triangulation shadow effect is to use multiple sources with a normally incident single receive triangulation configuration. Each of the sources illuminates the test surface from different incident angles. If one source is occluded, or otherwise blocked, from an area of the test surface, there is a high probability that the other source will be able to illuminate that area. To capture height information, the receive sensor acquires images from each of the sources serially and then combines the results of the multiple height images to ensure all areas of the test surface contain valid height data. Typically, this will require the sensor to be stationary and will require a separate image to be acquired for each of the sources. One disadvantage to this approach is that it requires multiple image acquisition cycles to generate a single height image, which slows down the overall acquisition process when compared to a sensor that uses a single source. Implementation of multiple source white light phase triangulation sensors requires the sources to be turned on one at a time so that the image from one source, followed by acquisition of an image from another source can be acquired in sequence by the receive detector. This operation will typically require two or more image acquisition cycles of the sensor to acquire height data from all areas of the image.
Providing a multi-source sensor for three dimensional imaging using phased structured light that does not have the associated cost or speed penalty that is present in the current state of the art for multiple art phase profilometers would represent a useful advance to high-speed three-dimensional inspection.